Worm gearing is well known and is used in installations where a driving shaft and a driven shaft must be at right angles to one another. The worm is of cylindrical outline in simplest form and has one or more thread turns or spiral teeth thereon. A meshing worm gear has a plurality of appropriately twisted teeth having tooth flank surfaces, generally of an involute curved shape, which are conjugate to the worm.
The instantaneous contact between a worm and a worm gear is at a rather limited area where the worm is tangent to the gear. The area of contact is quite limited, generally being aong a full line on one tooth and a very short line on an adjacent tooth. Consequently, only a limited amount of power can be transmitted between a worm and worm gear.
Efforts have been made to provide greater contact between a worm and a worm gear and thereby to permit greater transmission of power by shaping the worm somewhat in the nature of an hourglass. Manufacture of such worms is quite a complicated process, requiring precise control of the relative positions of the worm blank and of a cutting tool as the worm blank is turned. It is not possible to form hourglass worms by a thread rolling process as is possible with straight worms.